GB-A-1604634 describes such a system in which a control unit normally supplies a nominal 12 volts to the supply lines. In the absence of a fire (when no detecting device has responded), there is hardly any current drain on the lines. When a fire is detected by any of the detecting devices, more line current is drawn by the detecting device and this is seen (by the control unit) as a fire detection signal. The control unit then responds by changing the supply line voltage to a nominal 24 volts, thereby actuating the alarm devices. As the alarm devices are only actuated by a voltage of (e.g.) at least 17 volts, they do not sound an alarm when there is only 12 volts on the line. The detecting devices and the alarm devices can therefore operate on the same line pair to save wiring.
Despite the latter advantage (economy of wiring), there are disadvantages with such a single line pair system. The total current drain is negligible when the system is in its normal operating state (i.e. no fires are detected) and each detecting device draws only a fraction of a milliamp (even if tens of detecting devices are connected on a supply line, the total current drain is extremely small). However, as more and more detecting devices are triggered by a spreading fire, the total current drain will then be appreciable and this adds to the current drain imposed by the actuated alarm devices and it serves no useful purpose. Thus, whilst the system of GB-A-1,604,634 may work adequately when only a few detectors are employed, there is clearly a serious disadvantage when a multiplicity of detecting devices are present in the system and a fire is spreading through a region thereby actuating an increasing number of detecting devices. For example, spreading smoke can quickly actuate many smoke detectors.
There are clearly defined rules for safety that require, for example, that the battery capacity should be capable of sustaining operation of the complete system, in its quiescent state, for twenty four hours and for 30 minutes longer with all detecting and alarm devices actuated. This means that a large battery is required, along with an equally large battery charger (to maintain the battery in good condition). Therefore, the potential current drain imposes the need for an expensive and bulky battery and battery charger system. It also limits design, since bulky components are always a problem to accommodate in casings.
Various prior art attempts to provide fire alarm systems of the two-wire supply type will now be described.
GB-A-1,491,222 discloses an alarm system of the two-wire supply type, wherein detecting devices and alarm devices are connected in parallel across a single pair of lines which are monitored and conditioned by a central control unit. The control unit applies to the lines a d.c. supply with one polarity to which only the detecting devices are responsive (since the alarm devices are fitted with blocking diodes). In the event that the control unit senses a fire detection signal from a detecting device, the control unit applies to the lines a d.c. supply with the opposite polarity whereby the alarm devices are made to operate. This system enables both the detecting devices and the alarm devices to be supplied by the same line pair, and this is economic with wiring. A fire detection signal is signalled by a current flow in the detecting device and circuit means are provided (a blocking diode) to limit current flow in the detecting device when subjected to d.c. with opposite polarity. Whilst this means that a lower capacity standby battery may be used, a disadvantage is that special circuit means (two pole switching) is required in the control unit to reverse the supply polarity and special circuit means is required in the detecting devices to keep their alarm indicators on when the supply transitions through zero when the polarity is reversed. Moreover, the blocking diode would not serve the same purpose in a unipolarity system where the voltage is increased, from say 12 to 24 volts, to operate alarms and there is no change in polarity.
GB-A-2,281,995 discloses an alarm system which has some similarity to that in GB-A-1,491,222 except that (in the event of an alarm), an oscillator cyclically reverses the polarity to the detection and alarm devices so as to allow the presence of a fire detection signal from a detecting device to be periodically verified or determined by the control unit. This is advantageous when it is necessary to operate alarm devices on a second pair of lines, in response to a fire detection signal from a detecting device on a first pair of lines. With this system, it is possible to determine whether or not a detecting device has operated on the second pair of lines, while the alarm devices are being cyclically operated and thereby determine, for example, the spread of fire or smoke. However, the disadvantages attributed to GB-A-1,491,222 also generally apply to GB-A-2,281,995.
British Patent Application 9711745.1 discloses an alarm system in which detecting devices and alarm devices are connected in parallel across a single pair of lines monitored and conditioned by a central control unit. In a preferred embodiment the control unit applies to the lines a current limited first supply condition of 24 volts to which only the detecting devices are responsive. A fire detection signal is generated when a detecting device signal places a resistor across the lines so that the line voltage is reduced to 15 volts. At this voltage, the detecting devices retain their state but do not detect, so that further detecting devices cannot introduce further loads across the line. On registering an alarm from a detecting device the control unit first applies a second supply condition of 12 volts to the lines and secondly and additionally applies a sequence of voltage pulses to the lines which the alarm devices recognise and place themselves into an alarm state. When the second supply condition is applied the detecting devices are still in a "no detect" mode. The alarm devices are reset by applying a third supply condition whereby the supply voltage is reduced to between 5 and 6 volts at which level detecting devices still retain their state. By cyclically resetting the alarm devices, applying the first and second supply condition and sequence of voltage pulses the presence of an alarm signal from a detecting device can be periodically verified or determined by the control unit. A disadvantage of this system is that many common detecting devices such as smoke detectors require the first supply condition to be applied for typically several seconds before they have stabilised sufficient to detect smoke. This severely constrains the temporal operation of the alarm devices when seeking to periodically verify or determine the operation of a detecting device; for instance the output of an audible alarm device would be noticeably interrupted.
In another known alarm system, detecting devices and alarm devices are connected in parallel across a single pair of lines monitored and conditioned by a central control unit and the control unit applies (to the lines) a current limited first supply condition in the range 17 to 24.5 volts to which only the detecting devices are responsive. On registering a fire detection signal from a detecting device the control unit first applies a second supply condition in the range 25 and 31 volts to the lines in which range the alarm devices are made to operate. A sequence of pulses may then be superimposed on the supply whereby detecting devices which recognise the sequence will automatically inhibit themselves from signalling an alarm thereby limiting drain. The first and second supply conditions may be cyclically applied to the lines so that the presence of a fire detection signal from a detecting device to be periodically verified or determined by the control unit. The disadvantage of this system is that the control panel must include circuit means for generating a pulse sequence and detecting devices must include means for decoding the pulse sequence, both of which increase cost and complexity.